Fiber bleaching



' Patented Oct. 20, 1931 UNITED STA JOHN Known, or

GHICAGO, ILLINOIS FIBER BLEAGHING .No Drawing.

This invention relates to the bleaching of u paper and other fiberpulps, and among other ob ects aims to reduce the time and cost fbleaching and to minimize loss of fiber and fiber strength.

The invention may best be disclosed b reference to an illustrativemethod emb ying the invention and described in the followingspecification. The inventive method may ad- 10 vantageously be describedas applled to the. bleaching of kraft or sulphate pulp, slnce it isconsidered particularly difiicult to bleach.

The kraft pulp is supplied in slush form of a consistency which-mayadvantageously be 5 about ten per cent. The process may be carried oninany appropriate apparatus provided with agitating means, means forheating the pulp both under and without pressure. It will be obvious,however, that the various steps of the process need not be carried outin a single piece of apparatus. While specially designed apparatus maybe advantageous under certain conditions the process may be carried outin existing apparatus commonly employed in pulp treatment.

The unit proportions of treating substances used in the illustrativemethod will be based as is the usual practice, upon one ton of kraftpulp, air-dry weight. Into the apparatus above referred to isintroducedan oxidizing agent, in this instance bleaching powder, in the amount offrom fifty to one hundred (50 to 100) pounds. This ingredient isadvantageousl prepared in liquor form consisting of one 1) pound ofbleaching powder per gallon of water. The quantity of bleaching powderused (between the limits above specified) depends upon the character ofthe water (whether it be hardor soft) and upon the character of thepulp, i. e. for example,

whether it is a hard cook or soft cook pulp-the less oxidizing agentbeing used when the fibers are well opened up. In the apparatus thebleaching liquor is furtherdiluted with a quantity of water which willbring the pulp, when added, to a consistency of from five tosix percent. (5% to 6%) of pulp.

By adding the pulp to the bleaching liquor (instead of vice versa)the'burning of the Application filed March. 29,

1929. Serial No. 351,127.

pulp, in spots, by the liquor is avoided. If the apparatusadmits of it,the contents are agitated while the pulp is being added or w ile thebleaching liquor is added, if the sequence of the additions be reversed.

The pulp is then agitated for one hour at a temperature of from forty toeighty degrees Fahrenheit (40 to 80 F), depending on thecharacter of thepulp, the igher temperature stimulating action and being advantageousfor pulp in which the fibers are not well opened up.

The strength of the bleachin liquor used in the foregoing treatment isre atively weak but it bleaches the pulp to a yellow color. Its 1 actionis not apparently primarily a bleaching one. It seems to attack thecoloring matter and the fiber encrusting substances (with the possibleexception of ltch) and thus prepares the pulp for the e ectivedissolving or removal by the next ensuing treatment, of those encrustingsubstances which have not already been eliminated.

In preparation for such treatment the bleaching liquor is withdrawn andthe pulp 7 washed. It does no harm and possibly is advantageous to havea trace of the bleaching liquor remain in the pulp. This simplifies andshortens the washing of the pulp.

Into the same apparatus, assuming it to be adapted for cooking under alow steam pres sume (and from which the pulp may have been removed) analkaline cookin liquor, in this instance comprising the fol owingingredients, is added:

10 pounds caustic potash. 100 pounds soda ash.

5 pounds sodium perborate BO3-4 o i i Enough water is added to bring theconsistency of the pulp, when added, to five or six per cent. (5% or 6%)of pulp. The above named, ingredients are preferably added in 'the ordernamed, L e., with the perborate last.

The pulp is then added (again while the contents are agitated if that bepossible) and the batch cooked andagitated for from thirty (30) minutesto one (1) hour (depending on the character of the pulp) at from two tofive (2 to 5) pounds steam pressure.

This treatment apparently dissolves or removes the fiber encrustingsubstances with out causin substantial lossof fiber or fiber strength.to preserve the fiber strength, resulting in a qualit and character offiber particularl suitab e for rayon and high grade alpha oe lulosep.

The liquor employed is then drained off and may be reclaimed and usedagain if restored to strength by addition of the chemicals exhausted.The pulp is then washed.

The ulp is now in condition where it will respond effectively tobleaching treatment.

I have found that satisfactory results may also be obtained if in theforegoing cooking treatment five (5) pounds of caustic potash and fromforty to one hundred pounds of sodium silicate (depending, as before, oncharacter of fiber) be used in place of the caustic potash, soda ash andperborate,other proportions and details of treatment remainin the same.Where appropriate, as with jack and southern pine and other highlyresinous woods, such modified treatment commends itself because of thecheapness of the chemicals. If traces of silica remain in the pulp, thelatter treatment would probably not be satisfactory if the pulp were tobe used for rayon.

In the final bleaching treatment, a bleaching agent, in this caseconsisting of fifty pounds of bleaching powder in liquor form (aconcentration of one (1) pound of bleaching powder to a gallon ofwater), is introduced into an agitating vessel and water added to bringthe conslstency of the pulp, when added, to from five to six per cent.(5% to 6%). The washed pulp is then introduced and the contents agitatedat a temperature of from forty to sixty degrees Fahrenheit (40 to F.) Asolution containing a reducing bleaching agent, in this instance three(3) pounds of oxalic acid and one (1) pound of common salt (NaCl), isalso added but such addition can advantageously be deferred until theulp and chlorine liquor have been agitat a short while. The totalagitation and treatment should consume one (1) hour.

The interaction between the three chemicals in this treatment is notdefinitely understood but the eifect of the treatment is to produce adeadwhite pulp. Treated with the chlorine liquor alone, the pulp, whilebleached so that it can be termed bleached pulp and used commerciallyfor many purposes as bleached pulp, is not dead white.

Apparently the oxalic acid acts only as a reducing or bleaching agent,but reacts with the chlorine (or perhaps there is a catalytic action) tostimulate the bleaching action and to carry it beyond the point whichcould be reached by successive treatments of the chlorine liquor and theoxalic acid and salt, of the strengths referred to. Apparently the heperborate seems to cooperate oxalic acid finally neutralizes thechlorine because the pulp requires little washing to bring 1t to acondition where there is no objectionable after effect, such as chlorinediscoloratlon. The action of the salt is not fullyunderstood,-apparently it reacts to temper the harsh characteristics ofthe oxalic acid, making it practically available in a bleaching Iprocess of this character.

After treatment for one hour and subsequent washin the pulp is a deadwhite color and has su stantially' its original fiber strength. Althoughthe entire process consumes a relatively short time bleaching has beenaccomplished without harsh treatment 'of the pulp and the loss of fiberand fiber strength has therefore been minimized.

Obviously the invention is not limited to the precise chemical agentsand other details of the illustrative method. With pulps which respondmore readily to removal of encrusting substances or to bleaching, thelength of treatment or strength of chemicals ma be substantially variedor even some of the chemicals omitted,

or changed, depending upon the character and structure of the fiber,which characteristics are well known as regards -most fiber yieldingmaterials.

. I claim:

1. The method of bleachin kraft pul to a dead white color which is caracterize by cooking the pulp to free the fibers from the encrustinsubstances and then treating the pulp wit a bleaching agent containingoxalic acid and sodium chloride.

2. The method of bleaching kraft pulp which is characterized by treatingthe pulp to w free the fiber from encru-sting matter and then bleachingthe pulp by treatment with chlorine liquor, oxalic acid, and sodiumchloride.

3. The method of bleaching kraft pulp which is characterized by treatingthe pulp with chlorine liquor and then cooking it in an alkalinesolution to remove the fiber encrusting substances, and then bleachingthe pulp with chlorine liquor and oxalic acid in the presence of sodiumchloride.

In testimony whereof, I have signed my name to this specification.

JOHN NEUMANN.

CERTIFICATE or connection Patent No. 1,828, 333.- Granted October 20,1931, w

JOHN NEUMANN.

It is hereby certified thaterror appears m the printed specification ofthe above numbered patent requiring correction as follows; Page 2, line58; after the vvord "acts" insert the word not; and that the saidLetters Patent should be read with this correction therein that the samemay conform to the record of the case in the Patent Office.

Signed and sealed this 15th day of December, A. D. 1931.

. I M. J. Moore, (Seal) Acting Commissioner of Patents.

